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Subject[PATCH 2/4] mm: migrate: Close race between migration completion and mprotect
Date
A migration entry is marked as write if pte_write was true at the time the
entry was created. The VMA protections are not double checked when migration
entries are being removed as mprotect marks write-migration-entries as
read. It means that potentially we take a spurious fault to mark PTEs write
again but it's straight-forward. However, there is a race between write
migrations being marked read and migrations finishing. This potentially
allows a PTE to be write that should have been read. Close this race by
double checking the VMA permissions when migration completes.

Signed-off-by: Mel Gorman <mgorman@suse.de>
---
mm/migrate.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/mm/migrate.c b/mm/migrate.c
index f78ec9b..f59b5de 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -146,7 +146,9 @@ static int remove_migration_pte(struct page *new, struct vm_area_struct *vma,
pte = pte_mkold(mk_pte(new, vma->vm_page_prot));
if (pte_swp_soft_dirty(*ptep))
pte = pte_mksoft_dirty(pte);
- if (is_write_migration_entry(entry))
+
+ /* Recheck VMA as permissions can change during migration started */
+ if (is_write_migration_entry(entry) && (vma->vm_flags & (VM_WRITE)))
pte = pte_mkwrite(pte);
#ifdef CONFIG_HUGETLB_PAGE
if (PageHuge(new)) {
--
1.8.4.5


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